The present invention pertains generally to implantable medical devices and, more particularly, to implantable medical devices fabricated of self-supporting laminated films fashioned into geometric configurations adapted to specific medical uses. More particularly, the present invention relates to metal films, foils, wires and seamless tubes, with increased mechanical properties, which are suitable for use in fabricating implantable endoluminal grafts, stent-grafts and stent-graft-type devices. More specifically, the present invention comprises endoluminal grafts, stent-grafts and stent-graft-type devices that are fabricated entirely of self-supporting laminated films, foils, wires or seamless tubes made of biocompatible metals or of biocompatible materials which exhibit biological response and material characteristics substantially the same as biocompatible metals, such as for example composite materials.
As opposed to wrought materials that are made of a single metal or alloy, these inventive materials are made of at least two layers formed upon one another into a self-supporting laminate structure. Laminate structures are generally known to increase the mechanical strength of sheet materials, such as wood or paper products. Laminates are used in the field of thin film fabrication also to increase the mechanical properties of the thin film, specifically hardness and toughness. Laminate metal foils have not been used or developed because the standard metal forming technologies, such as rolling and extrusion, for example, do not readily lend themselves to producing laminate structures. Vacuum deposition technologies can be developed to yield laminate metal structures with improved mechanical properties. In addition, laminate structures can be designed to provide special qualities by including layers that have special properties such as superelasticity, shape memory, radio-opacity, corrosion resistance etc.
Metal foils, wires and thin-walled seamless tubes are typically produced from ingots in a series of hot or cold forming steps that include some combination of rolling, pulling, extrusion and other similar processes. Each of these processing steps is accompanied by auxiliary steps that include cleaning the surfaces of the material of foreign material residues deposited on the material by the tooling and lubricants used in the metal forming processes. Additionally, chemical interaction with tooling and lubricant materials and ambient gases also introduces contaminants. Some residue will still usually remain on the surface of the formed material, and there is a high probability that these contaminating residues become incorporated during subsequent processing steps into the bulk of the wrought metal product. With decreasing material product size, the significance of such contaminating impurities increases. Specifically, a greater number of process steps, and, therefore, a greater probability for introducing contaminants, are required to produce smaller product sizes. Moreover, with decreasing product size, the relative size of non-metal or other foreign inclusions becomes larger. This effect is particularly important for material thicknesses that are comparable to the grain or inclusion size. For example, austenitic stainless steels have typical grain sizes on the order of magnitude of 10-100 micrometer. When a wire or foil with a thickness in this range is produced, there is significant probability that some grain boundaries or defects will extend across a large portion or even across the total thickness of the product. Such products will have locally diminished mechanical and corrosion resistance properties. While corrosion resistance is remedied by surface treatments such as electropolishing, the mechanical properties are more difficult to control.
The mechanical properties of metals depend significantly on their microstructure. The forming and shaping processes used to fabricate metal foils, wires and thin-walled seamless tubes involves heavy deformation of a bulk material, which results in a heavily strained and deformed grain structure. Even though annealing treatments may partially alleviate the grain deformation, it is typically impossible to revert to well-rounded grain structure and a large range of grain sizes is a common result. The end result of conventional forming and shaping processes, coupled with annealing, typically results in non-uniform grain structure and less favorable mechanical properties in smaller sized wrought metal products. It is possible, therefore, to produce high quality homogeneous materials for special purposes, such as micromechanical devices and medical devices, using vacuum deposition technologies.
In vacuum deposition technologies, materials are formed directly in the desired geometry, e.g., planar, tubular, etc. The common principle of the vacuum deposition processes is to take a material in a minimally processed form, such as pellets or thick foils (the source material) and atomize them. Atomization may be carried out using heat, as is the case in physical vapor deposition, or using the effect of collisional processes, as in the case of sputter deposition, for example. In some forms of deposition, a process, such as laser ablation, which creates microparticles that typically consist of one or more atoms, may replace atomization; the number of atoms per particle may be in the thousands or more. The atoms or particles of the source material are then deposited on a substrate or mandrel to directly form the desired object. In other deposition methodologies, chemical reactions between ambient gas introduced into the vacuum chamber, i.e., the gas source, and the deposited atoms and/or particles are part of the deposition process. The deposited material includes compound species that are formed due to the reaction of the solid source and the gas source, such as in the case of chemical vapor deposition. In most cases, the deposited material is then either partially or completely removed from the substrate, to form the desired product.
The rate of film growth is a significant parameter of vacuum deposition processes. In order to deposit materials that can be compared in functionality with wrought metal products, deposition rates in excess of 1 micrometers/hour are a must and indeed rates as high as 100 micrometers per hour are desirable. These are high deposition rates and it is known that at such rates the deposits always have a columnar structure. Depending on other deposition parameters, and most importantly on the substrate temperature, the columns may be amorphous or crystalline but at such high deposition rates microcrystalline structure development can be expected at best. The difficulty is that the columns provide a mechanically weak structure in which crack propagation can occur uninhibited across the whole thickness of the deposit.
A special advantage of vacuum deposition technologies is that it is possible to deposit layered materials and thus films possessing exceptional qualities may be produced (c.f., H. Holleck, V. Schier: “Multilayer PVD coatings for wear protection”, Surface and Coatings Technology, Vol. 76-77 (1995) pp. 328-336). Layered materials, such as superstructures or multilayers, are commonly deposited to take advantage of some chemical, electronic, or optical property of the material as a coating; a common example is an antireflective coating on an optical lens.
It has not been recognized until relatively recently that multilayer coatings may have improved mechanical properties compared with similar coatings made of a single layer. The improved mechanical properties may be due to the ability of the interface between the layers to relieve stress. This stress relief occurs if the interface provides a slide plane, is plastic, or may delaminate locally. This property of multilayer films has been recognized in regard with their hardness but this recognition has not been translated to other mechanical properties that are significant for metal products that may be used in application where they replace wrought metal parts.
A technological step that interrupts the film growth results in discontinuous columns and prevents crack propagation across the entire film thickness. In this sense, it is not necessary that the structure consist of a multiplicity of chemically distinct layers, as it is common in the case of thin film technology where multilayers are used. Such chemical differences may be useful and may contribute to improved properties of the materials.
In its simplest form, the inventive process consists of the steps of providing a substrate, depositing a first layer of material on the substrate, depositing a second layer of material on the first layer of material and optionally removing the layered material from the substrate. The last step is necessary in the case of making foils and seamless tubes but would be omitted in the case of making wires. In this latter case, the substrate itself is a thin wire that becomes part of the final product. In more complex cases, the number of layers is more than two. There is no limitation regarding the number of layers and regarding the thickness of each layer.
As used in this application a “layer” is intended to mean a substantially uniform material limited by interfaces between it and adjacent other substantially homogeneous layers, substrate, or environment. The interface region between adjacent layers is an inhomogeneous region in which extensive thermodynamic parameters may change. Different layers are not necessarily characterized by different values of the extensive thermodynamic parameters but at the interface, there is a local change at least in some parameters. For example, the interface between two steel layers that are identical in composition and microstructure may be characterized by a high local concentration of grain boundaries due to an interruption of the film growth process. Thus, the interface between layers is not necessarily different in chemical composition if it is different in structure.
It is necessary to provide for good adhesion between the layers and this is usually achieved by providing for a relatively broad interface region rather than for an abrupt interface. The width of the interface region may be defined as the range within which extensive thermodynamic parameters change. This range can depend on the interface area considered and it may mean the extent of interface microroughness. In other words, adhesion may be promoted by increased interface microroughness between adjacent layers.
By providing for a layered structure, the inventive materials consist of a controlled maximum size of grains and columns as extended defects in the direction of the film growth (perpendicular to the layers). This limit of the grain or defect size results in materials that have increased mechanical strength and particularly increased toughness compared to their non-laminated counterparts, both deposited and wrought materials. In addition, limiting the extent to which defects and grain boundaries reach across the laminate, corrosion resistance is also improved.
Laminated materials will have additional advantages when chemical compositions of the layers are chosen to achieve special properties. For example, a radiopaque material such as Ta may form one layer of a structure while other layers are chosen to provide the material with necessary mechanical and other properties.
Without limiting the scope of application of the present invention, the following are specific examples of products or devices which may be fabricated using the laminated film and process of the present invention: 1) an implantable graft fabricated of laminated films of biocompatible metals or biocompatible materials which exhibit in vivo biological and mechanical responses substantially the same as biocompatible metals (hereinafter referred to as “metal-like materials”); 2) an implantable stent-graft device in which a structural component, or stent, and a graft component are each fabricated of laminated films of metal or metal-like materials; 3) an implantable stent-graft-type device in which a structural support, such as a stent, defines openings which are subtended by a web, with both the stent and the web being formed as a single, integral, laminated film or tubular structure and fabricated of metals or of metal-like materials, this particular embodiment is hereinafter referred to as a “web-stent;” and 4) planar films, sheets or foils made of laminated biocompatible metals or biocompatible materials, suitable for use as medical tissue patches, aerospace surfaces, such as leading edges of aircraft wings or helicopter rotors, or as active surfaces on tail rudders or wing flaps of aircraft.